This invention relates to the manufacture of fatigue-resistant articles that intentionally have internal cavities present, and, more particularly, to the manufacture of hollow fan blades for aircraft gas turbine engines.
In a conventional aircraft gas turbine (jet) engine, air is drawn into the front of the engine and compressed by an axial flow compressor. The compressed air is mixed with fuel, and the mixture is ignited to produce a hot exhaust gas. The exhaust gas flows through a turbine that drives the axial flow compressor, and the exhaust gases are then exhausted through the rear of the engine to drive the engine and aircraft forward. Additional thrust may be generated by using the exhaust gases to turn a large-diameter fan that draws additional air, sometimes termed bypass air, through a ducted fan that surrounds the engine core.
The fan employs a large number of fan blades that extend outwardly from a central shaft. These fan blades act much like propeller blades to drive the bypass air flow rearwardly, generating thrust through the reaction between the fan blades and the bypass air flow. In a large aircraft engine such as those used in airliner jumbo jets, the fan blades may be several feet long.
The fan blades must be strong and also quite light, because they turn rapidly on the central shaft and generate a great deal of centrifugal loading on the central shaft. The heavier the fan blades, the heavier must be the shaft, bearings, support structure, etc. Additionally, the fan blades must be resistant to various types of damage that can occur during their use. The fan blades must resist erosion of particles in the air, damage from impacts such as ingested particles and birds, and accumulations of fatigue damage.
One approach to the design of fan blades is to manufacture the fan blades from a relatively light weight alloy such as a titanium alloy. Weight can be saved by making the fan blade hollow, with reinforcing ribs extending internally between the sides of the surface skins of the fan blade. Techniques are known for manufacturing such hollow fan blades of titanium alloys, with well defined internal cavities intentionally present to reduce the weight of the fan blade.
An important consideration which limits the life of hollow fan blades is fatigue. During engine operation, the fan blade is loaded in a generally axial direction by centrifugal force. There is additionally a variable loading superimposed on the constant component of the loading as the fan blade rotates past struts and other structure in the fan duct. The combined constant and variable components of the loading produce fatigue cracks in the fan blade. If any one fatigue crack in a fan blade propagates to a sufficiently large length, it causes the fan blade to fail.
The development of fatigue cracks generally occurs by a two-stage mechanism involving first initiation of the fatigue crack at a surface and then growth of the fatigue crack through the body of the fan blade. Various techniques are used to reduce fatigue crack initiation and growth. The techniques which rely upon metallurgical processing usually involve specially selected surface treatments and microstructures to limit fatigue crack initiation, and other specially selected interior microstructures to limit fatigue crack growth. The microstructures that minimize fatigue crack initiation are typically different from those that minimize fatigue crack growth.
Applying these principles to fatigue crack control in fan blades, the fan blade will usually be produced to have a particular microstructure throughout its body that is resistant to fatigue crack growth. There are a number of processes that can be subsequently applied to the external surfaces of the fan blades to alter their structure to be more resistant to fatigue crack initiation. However, where the fan blade is hollow, the interior of the fan blade is inaccessible to preferential surface heaters and mechanical peeners that are often used on the external surfaces. Since they are not treated to minimize fatigue crack initiation, these internal surfaces of the cavities become preferential sites for fatigue crack initiation during service, leading to early failure of the fan blade.
There is a need for an improved approach to the manufacture of hollow fan blades and other types of articles that are subjected to fatigue during service. Such an improved approach must be compatible with the other manufacturing steps of the hollow article, and also not adversely affect other properties of the hollow article such as strength, corrosion resistance, impact resistance, etc. The present invention provides such an approach, and further provides related advantages.